In order to understand the inconveniences that are meant to be remedied by the present invention, this description continues by illustrating the structural and functional configuration of this type of known locks.
This type of “push-push” locks are provided with a roto-translating pin that acts as engagement means for the lid, having a basically rectangular head that is inserted and engaged in a box-shaped housing on the internal side of the lid, characterised by the fact that it is provided with a rectangular slot that gives access to the internal compartment of the said housing with higher width compared to the rectangular slot.
The said roto-translating pin is inserted inside a housing and guiding chamber, in which it is free to slide axially and rotate, being constantly subject to the force of an ejection spring compressed during the closing of the lid.
The said roto-translating pin is matched with the housing and guiding chamber in such a way that a 90° rotation of the pin is determined around its longitudinal axis during each forward or backward travel, and is repeated in inverse direction during the next ejection or forward travel.
Evidently, at the end of the ejection travel of the pin, the rectangular head must have a position aligned with the slot through which the engagement head enters and exits the engagement housing, while at the end of the forward travel of the pin, the rectangular head must have an orthogonal position with respect to the slot in order to remain engaged inside the slot.
Finally, it must be said that a special cam—known as heart-shaped cam—is provided on one side of the roto-translating pin, at the opposite end with respect to the engagement head, which is in turn engaged with a flexible tappet.
Cams of this type are commonly used in the so-called click pens, in which every pressure of the pen button alternatively causes the stable release or the stable retraction of the pen tip.
The said heart-shaped cam comprises two parallel tracks joined at the ends to form a closed circuit, in which the said tappet slides cyclically in one direction only.
It must be noted that the bottom surface of one of the said tracks has a series of ascending inclined planes, while the other track has a series of descending slides and steps, so that the tappet that slides inside the circuit moves forward in a forced direction along the descending slides and steps and then inverts the travel direction along the ascending inclined planes.
The heart-shaped cam definition derives from the fact that the shape of the closed circuit formed by the said pair of parallel tracks suggests the shape of a heart, being characterised by a lower point and a V-shaped central upper cavity.
More precisely, the said tappet is alternatively engaged and released from the said V-shaped cavity at every small travel of the heart-shaped cam, which obviously follows the alternated travels of the said roto-translating pin.
When the lid is closed, the tappet is firmly engaged with the heart-shaped cam, so that also the pin is held in backward position, in spite of the force of the ejection spring.
If the user presses the lid, the roto-translating pin moves back shortly inside the housing and guiding chamber, together with the heart-shaped cam, with consequent release of the tappet, in such a way that the roto-translating pin moves forward under the force of the ejection spring.
As mentioned earlier, at the end of the ejection travel of the roto-translating pin, the engagement head is not engaged to the lid, thus allowing the user to open it.
When the lid is closed and pushed against the engagement head, the roto-translating pin is pushed inside the housing and guiding chamber again, in such a way that at the end of the backward travel the tappet is re-engaged automatically and firmly inside the cavity of the heart-shaped cam.
As mentioned earlier, at the end of the backward travel of the roto-translating pin, the engagement head is engaged inside the engagement housing on the internal side of the lid, thus preventing the user from opening it.
The first inconvenience of this type of push-push locks is represented by the excessive overall dimensions in parallel direction to the longitudinal axis of the said roto-translating pin.
The said inconvenience is caused by the fact that the heart-shaped cam is located in the back in alignment with the roto-translating pin, whose length is therefore added to the length of the heart-shaped cam and relevant flexible tappet.
This dimension affects the selection of the inclination angle of the helicoidal thread that determines the roto-translation of the roto-translating pin.
As a matter of fact, threads with high inclination angles should be used in order to reduce the force necessary to push the roto-translating pin inside the housing and guiding chamber.
The higher the inclination angle, the longer the travel of the roto-translating pin will be, in order to allow 90° rotation of the engagement head.
This means that small inclination angles must be used to reduce the dimensions of the current locks, with the inconvenience that the user must push the lid hard.
Another inconvenience consists in the fact that the roto-translating pin is subject to an unbalanced force system that favours the creation of considerable friction able to prevent the free roto-translation of the pin.
This second inconvenience is due to the fact that the said heart-shaped cam is fixed to one side of the roto-translating pin, on which the flexible tappet, which slides against and along the tracks of the heart cam, discharges its unbalanced force.
Another inconvenience of this type of known push-push locks consists in the difficult assembly due to the mutual position of the roto-translating pin and the heart-shaped cam with the relevant flexible tappet.